1. Field of the Invention
This invention relates to an apparatus and method for registering superimposed images in an image forming apparatus. In particular, this invention relates to an apparatus and method for registering a plurality of component images formed by a tandem color image forming apparatus.
2. Discussion of the Related Art
In an image forming apparatus in which a plurality of separately formed component images are required to be superimposed upon one another, such as a color copier, it is extremely important to ensure that the proper adjustments are made to the apparatus so that the component images are precisely registered and superimposed.
Misregistration is a system level composite error in the relative positioning of one component image with respect to the other component images and resulting in the component images not being properly superimposed. Misregistration may be broken down into several types including lateral direction offset, process direction offset, skew, lateral magnification, and bow. Any of these types of misregistration may be present in any instant of system operation.
In a tandem image forming apparatus; e.g., one having a plurality of developing stations positioned along an intermediate or transport belt, there are several possible sources of misregistration. First, there may be lateral movement or stretching of the intermediate belt relative to the developing stations resulting in lateral or directional mispositioning of the component images. Second, any of the plurality of optical elements in an image beam forming portion of the apparatus may become loose or be improperly adjusted, thus resulting in the occurrence of any or all of the above mentioned types of misregistration. Third, the component image forming stations may not be properly synchronized. Fourth, a photoreceptor drum in a component image forming station may not be properly positioned with respect to the image forming optics (commonly referred to as drum runout), causing a lateral magnification error. Fifth, the intermediate belt may be conical and cause a transport skew on the image. Sixth, a photoreceptor drum may be skewed with respect to the intermediate belt and the other photoreceptor drums to cause a skewed component image.
Any of the types of misregistration may be caused by more than one of the mentioned causes of misregistration, and any of the mentioned causes of misregistration may be responsible for causing more than one component of misregistration. Therefore, the prior attempts to correct one type of misregistration by controlling one of the causes of that type of misregistration will not necessarily eliminate that type of misregistration from occurring.
Extensive efforts have been made in attempts to eliminate lateral offset. One of the causes of lateral offset is lateral shifting of the intermediate belt. Thus, an approach taken to solve this problem has been to detect and compensate for lateral shifting of the belt. However, if any of a plurality of optical elements in the image beam forming portion of the apparatus are loose or not adjusted properly, the apparatus will exhibit lateral offset.
In U.S. Pat. No. 4,912,491, issued to Osamu Hoshino et al., a system-wide approach to avoiding misregistration is described in which each of the component image forming stations forms registration indicia which are superimposed upon one another on a transparent strip formed in an intermediate belt. A sensor is used to detect the positions of the registration indicia. By comparing the detected positions with predetermined target positions, it can be determined whether a misregistration error is present. A problem in using the approach taught in this U.S. Patent, is that the approach is very dependent upon the intermediate belt travelling at a constant speed. For example, if the belt speed has decreased due to stretching or slipping of the belt, the predetermined target positions for the registration marks would not coincide with the proper actual positions of the registration marks. Although it is conceivable that the belt speed could be detected and adjustments could be made to compensate for variations in the belt speed, such an approach is not practical because it is very difficult to detect the DC velocity of the belt to the degree of precision necessary for properly registering images.
A further problem arises because the registration marks formed on the belt are positioned outside the image forming zone, making it difficult to reduce the effects of composite bow error introduced by each of the image forming stations.